CHAP. XXXII 
ERUPTIVE ROCKS OF THE MIDLANDS 
103 
4. Coalbrookdale Coal-field . — In this interesting district a sill of rather 
finely crystalline olivine-dolerite, which is estimated to be nearly 200 feet 
thick, is traceable from near Little Wenlock for three miles to the north, 
intercalated between the Carboniferous Limestone and the Silurian rocks 
underneath. It appears to underlie the western part of the Coal-field, for it 
is exposed by denudation in several valleys between Little Wenlock and 
Great Dawley. Owing to the thinning out of the Carboniferous Limestone 
in an easterly direction, the sill gradually comes to have the Millstone Grit on 
its upper surface, and at one point is represented on the Geological Survey 
map as even intruded into the Coal-measures. Here again we have an 
intrusive sheet of later date than at least the earlier part of the Coal- 
measures, and no evidence of any superficial outflow of volcanic material. 1 
5. South Staffordshire Coal-field . — This district, in respect to its igneous 
intercalations, has been much more fully examined and described than any 
of the others. It forms the subject of an exceedingly able memoir by Jukes, 
who carefully studied its geology and delineated it on the maps and sections 
of the Geological Survey. Since his time the rocks have been studied 
microscopically, but no material facts regarding the stratigraphy have been 
obtained in addition to those which he patiently collected and generalized 
upon. 2 
This coal-field is above 20 miles long and 5 miles broad. Its strata rest 
unconformably on Upper Silurian strata, which, as part of the ancient ridge 
or island already referred to, project here and there from amidst the Coal- 
measures. The boundaries of the field on the east and west sides are chiefly 
made by faults which bring down Permian and Triassic formations against 
the Carboniferous strata. 
Throughout this coal-field sheets of igneous rock are abundant. 
1 11 the detailed account of them given by Jukes in his admirable essay 
on the South Staffordshire Coal-field, 3 he distinguished two kinds of 
igneous material — “basalt,” which comes out at the surface, and some- 
times overlies the Coal-measures in large cakes like that of the Eowley 
Hills, which extends for two miles in one direction and more than a mile 
m another; and “greenstone,” which burrows among the coal -bearing- 
strata, and gives off dykes and veins of “ white rock-trap.” There does not 
cl PPear, however, to be any essential difference in composition, age or origin 
between these contrasted kinds of igneous material. They not improbably 
ell belong to one series of extrusions, their distinctions being due rather to 
®inute structure of the intrusion which forms a long ridge on the east side, are described by Mr. 
p° r t, Quart. Journ. Geol. Soc. xxx. pp. 550, 551. 
The Coalbrookdale coal-field lias been described by Sir Joseph Prestwich, Trans. Geol. Soc. (2) 
V- p. 428 ; ami Prof. E. Hull, Quart. Jour. Geol. Soc. xxxiii. (1877), p. 629. The minute structure of 
Qy. 6 J j *ttle Wenlock is referred to by Mr. Allport, op. cit. p. 550. The ground is mapped on 
eet 61 N.E. of the Geological Survey, and its structure is shown on Sheet 54 of the Horizontal 
Sections. 
Jukes, “South Staffordshire Coal-field,” Mem. Geol. Surv. 2nd edit. (1859). The area is 
tni laced in Sheet 62 N. W. and S.W. of the Geological Survey, and is illustrated in Sheets 23, 
and 25 of the Horizontal Sections. 3 Op. cit. p. 117. 
